Preparation of Substituted Indoxyls and Aminoindoles

Significance

Reported is a variation of the classical Ugi reaction, in which the nitrilium ion generated by the addition of the isocyanide to the iminium ion is interrupted by an electron-rich arene. The authors hypothesized such a trapping to proceed by replacing the carboxylic acid component with a Brønsted or a Lewis acid (J. A. Deyrup et al. Tetrahedron 1969, 25, 1467). Screening of a variety of acids led to triflic acid A as the reagent of choice. When R⁴ ∂ H, the imino intermediates may be ­hydrolyzed under mild conditions to afford substituted indoxyls. The substrate scope was fairly well studied and the reaction proceeds in yields ranging from moderate to excellent.